An octopus has three hearts. All with jobs to do.

An octopus has three hearts with different jobs. Two push blood through the gills, where it picks up oxygen; the third sends it around the rest of the body. They are not three spare copies: they belong to the same circulatory system. A small working group that cannot skip its meeting. [1]

Another surprise: oxygenated blood is blue. The oxygen carrier is haemocyanin, a protein containing copper, rather than our iron-containing haemoglobin. Three hearts and different chemistry: before inventing an extraterrestrial, it is worth taking a look beneath the sea. [2]